EXPERIMENTS ON THE LIMITATION OF BIRD BREEDING DENSITIES - A REVIEW

The breeding densities of birds could be limited by resources, such as food and nest sites, or they could be held at a lower level by natural enemies, such as predators and parasites. In this paper, I review the experimental evidence for each of these limiting factors affecting bird breeding densities. Of 18 experiments involving winter food provision (mostly on tits, Paridae), 11 led to increased breeding densities compared with control areas. Of four involving summer food depletion (all on forest insectivores), none led to decreased breeding densities. In experiments with Red Grouse Lagopus l. scoticus, fertilizing areas of heather moor led to increased densities during a period of population increase but did not prevent a later decline. Of 32 studies on tree-cavity nesters, the provision of nestboxes led to increased breeding density in 30 (95%) studies, each involving one or more species of hole nesters. Of 15 experiments involving predator removal (mostly on ducks and gamebirds), at least 14 led to increased hatching success, four out of eight led to increased post-breeding numbers, and six out of 11 led to increased breeding density. In one experiment, the removal of stronglye parasites from a Red Grouse population prevented a cyclic decline on five out of five occasions. Taken together, these experiments confirmed that all main potential external limiting factors have affected breeding density in one bird species or another. They also confirmed that the same species has been limited by one factor in certain areas or years and by another factor in different areas or years.